1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which is adapted to change a desired air-fuel ratio to which the air-fuel ratio of a mixture supplied to the engine is controlled, depending upon an amount of variation in the rotational speed of the engine during execution of so-called lean-burn control.
2. Prior Art
An air-fuel ratio control system for internal combustion engines is conventionally known which performs so-called lean-burn control of controlling the air-fuel ratio of a mixture supplied to the engine to a value leaner than a stoichiometric air-fuel ratio when the engine is in a steady operating condition or in a gently accelerating condition.
The conventional air-fuel ratio control system of this kind includes one proposed e.g. by Japanese Patent Publication (Kokoku) No. 6-23553, according to which when predetermined conditions (hereinafter referred to as "the lean-burn control-permitting conditions") under which lean-burn control is permitted are fulfilled, the air-fuel ratio of a mixture supplied to the engine is changed to a predetermined desired lean air-fuel ratio, by progressively changing the desired air-fuel ratio of the mixture toward a leaner side by a predetermined amount, and then an amount of variation in the rotational speed (rotational speed variation amount) of the engine is detected. The detected rotational speed variation amount is compared with a predetermined desired lean value, and when the former is larger than the latter, the air-fuel ratio is controlled to a slightly richer value than the predetermined desired lean air-fuel ratio. That is, a degree of unstability of the engine operating condition is detected during execution of the lean-burn control, and when the detected degree of unstability is higher than a predetermined value set for preventing a misfire of the engine, the air-fuel ratio is enriched such that the degree of unstability becomes lower than the predetermined value.
In the proposed air-fuel ratio control system, however, the air-fuel ratio is controlled by the use of a correction amount which enriches the air-fuel ratio according to the rotational speed variation amount during execution of the lean-burn control, resulting in an undesirable increase in the amount of generation of NOx from the engine.
FIG. 19A shows an example of changes in the air-fuel ratio obtained by the lean-burn control of the conventional air-fuel ratio control system. As shown in FIG. 19A, the air-fuel ratio of a mixture supplied to the engine is changed in a leaning direction toward a predetermined desired lean air-fuel ratio. If the controlled air-fuel ratio is made leaner than an actual lean limit for stable engine combustion, the combustion of the engine becomes unstable as indicated by (1). Further, according to the conventional system, the correction amount of the air-fuel ratio applied at the actual lean limit for stable engine combustion is reset upon termination of the lean-burn control. Therefore, when the lean-burn control is subsequently started again after the air-fuel ratio has been once controlled to the stoichiometric air-fuel ratio due to a change in the operating condition of the engine (as indicated by (3)), the air-fuel ratio of the mixture is leaned to the predetermined desired lean value, which causes unstable combustion of the engine again (indicated by (4)). Further, when the engine combustion becomes unstable as indicated by (1), the air-fuel ratio is controlled to a richer value until the combustion state of the engine is stabilized. However, the enriched air-fuel ratio is still employed as indicated by (2) in FIG. 19A even after the combustion state of the engine has been stabilized, which results in generation of NOx. Further, in this case, even if the actual lean limit for stable engine combustion is shifted in a leaning direction as indicated by (2), the controlled air-fuel ratio is held at the enriched value, which hinders improvement of the fuel economy.
Further, in the conventional air-fuel ratio control system, the correction amount which enriches the desired air-fuel ratio according to the rotational speed variation amount during execution of the lean-burn control has no upper limit set therefor, so that when the lean limit for stable engine combustion becomes much richer due to a change in the environmental condition or the operating condition of the engine, the desired air-fuel ratio is set to an abnormally enriched value. As a result, the fuel economy is not improved to a satisfactory degree, and at the same time NOx is generated in abnormally increased amounts.
Further, according to the conventional air-fuel control system, the desired air-fuel ratio for the air-fuel ratio feedback control is not directly changed from the stoichiometric air-fuel ratio to the desired lean air-fuel ratio, but the desired air-fuel ratio is progressively changed to the desired lean air-fuel ratio, so as to prevent a shock caused by a drastic change in the air-fuel ratio.
However, in the course of the progressive change of the desired air-fuel ratio from the stoichiometric air-fuel ratio to the desired lean air-fuel ratio, a change in the engine torque is caused by the change in the air-fuel ratio, and this change in the engine torque can be erroneously judged to have been caused by an unstable combustion of the engine though the engine is in a stable combustion. As a result, the desired air-fuel ratio is corrected in an enriching direction, which hinders the desired air-fuel ratio from being smoothly changed to the leaning desired lean air-fuel ratio, so that a time period during which the air-fuel ratio assumes such a value as will cause a larger amount of generation of NOx becomes longer.
Still further, in the conventional air-fuel ratio control system, the air-fuel ratio is corrected in an enriching direction when the detected rotational speed variation amount exceeds a predetermined fixed threshold value. However, individual engines actually employed have respective peculiar combustion variation characteristics ascribable to manufacturing tolerances of the engines, etc. Therefore, if variations in the combustion state of the engines are each determined using the same threshold value and the desired lean air-fuel ratio is corrected based on results of the determination, the air-fuel ratio of a mixture supplied to the engine can deviate from an optimal value at which the driveability and fuel economy of the engine can be balanced.